Planar substrates, made from either glass (e.g. microscope slide) or optical polymer (e.g. Zeonex), are widely used in fluorescence-based optical chemical and biological sensors as a low-cost surface onto which multiple sensor or biorecognition sites can be patterned. Following excitation, the fluorescence from these sensor sites can be detected by placing a CCD- or CMOS-based imaging system below the substrate. However, this configuration results in low fluorescence collection efficiency due to a combination of the anisotropic nature of the fluorescence emission and the light-guiding behaviour of the substrate which results in a significant proportion of the emitted fluorescence being trapped within the substrate by total internal reflection (TIR). In addition, for many such configurations, the numerical aperture (NA) of the detection optics is low resulting in only a small fraction of the transmitted fluorescence being received by the detector. Given that this imposes a significant limitation on the optical detection performance of this sensor type, a number of strategies have been proposed to improve their optical detection sensitivity. However, a common feature of these strategies is a trade-off between the usability of the substrate for patterning and the enhancement in fluorescence collection efficiency achieved. For example, one such strategy involves the collection of a specific type of luminescence, fluorescence, and more specifically a type of fluorescence known as supercritical angle fluorescence (SAF) by integrating optical elements onto the top surface of the substrate. The SAF light typically propagates within the substrate at angles within the range of ˜61.5-75°. These known optical elements however can affect the continuous nature of the sensor's top surface which is not optimal.
There is therefore a need for a luminescence based sensor configured for optimal configuration of super critical angle light without affecting the configuration of the top surface of the sensor.